Mobile cellular communication systems have become of increasing importance, providing mobile users the security of being able to seek aid in case of trouble, allowing dispatching of delivery and other vehicles with little wasted time, and the like. Present cellular communication systems use terrestrial transmitters, such as fixed sites or towers, to define each cell of the system, so that the extent of a particular cellular communication system is limited by the region over which the towers are distributed. Many parts of the world are relatively inaccessible, or, as in the case of the ocean, do not lend themselves to location of a plurality of dispersed cellular sites. In these regions of the world, spacecraft-based communication systems may be preferable to terrestrial-based systems. It is desirable that a spacecraft cellular communications system adhere, insofar as possible, to the standards which are common to terrestrial systems, and in particular to such systems as the GLOBAL SYSTEM FOR MOBILE COMMUNICATIONS system (GSM), which is in use in Europe.
The GSM system is a cellular communications system which communicates with user terminals by means of electromagnetic transmissions from, and receptions of such electromagnetic signals at, fixed sites or towers spaced across the countryside. The GSM system is described in detail in the text The GSM System for Mobile Communications, subtitled A Comprehensive Overview of the European Digital Cellular System, authored by Michel Mouly and Marie-Bernadette Pautet, and published in 1992 by the authors, at 4, rue Elisee Reclus, F-91120 Palaiseau, France. Another text that describes the GSM system is Mobile Radio Communications, by Raymond Steele, published by Pentech Press, London, ISBN 07273-1406-8. Each fixed site or tower (tower) of the GSM system includes transmitter and receiver arrangements, and communicates with user terminals by way of signals having a bandwidth of 50 Mhz., centered on 900 Mhz., and also by way of signals having a bandwidth of 150 Mhz., centered on 1800 Mhz.
A cellular communication system should provide communications to a terrestrial or land-line exchange from a mobile user terminal, and the terrestrial system should be able to reach a particular mobile user terminal regardless of the location of the mobile terminal (regardless of which cell the mobile station occupies). In the GSM system, each fixed site transmits a unique control signal, including a Broadcast Control Channel (BCCH), which in turn includes System Information Messages, which in turn includes Mobile Country Code (MCC), Mobile Network Code (MNC), and Location Area Code (LAC). The MCC identifies the country in which the fixed site or cell Is located, the MNC identifies the "owner," which is the administrator or provider which maintains the particular cell which transmits the BCCH, and the LAC identifies the particular cell which makes the transmission. These three signals are referred to together as Location Area Identity (LAI). The mobile user terminal stores the Location Area Identity (LAI) from a previous transmission, so the previous LAI is available to the user terminal at all times.
The provider must know the current location of the user terminal, in order to provide service to someone seeking access to the mobile user terminal without polling every possible cell in which the user terminal might be located. In general, if the user terminal is not located in the same cell as that from which it made its last transmission, it may be anywhere in the system. In the GSM system, if a mobile station moves from a location near a first fixed site to another site, without more, service would be lost, because in the overlap region between cells, the frequency of the transmissions from a second fixed site, being at different frequencies from those of the first fixed site, would not be recognized by the mobile user terminal.
When a GSM user terminal is energized in order to make a transmission, it first scans frequency to identify all control carriers or fixed sites which it can receive. It then selects the "best" one, which may be the one providing the greatest signal strength, or best signal-to-noise ratio, or the like. It then examines the LAI of that particular carrier, and receives the new LAI from the selected fixed site. The mobile station then compares the stored LAI with the newly received LAI. If the mobile user terminal is still within the broadcast area of the same fixed site as that with which it last communicated, the new and stored LAI will match. Since the mobile user terminal has not moved since the last communication, the overall system already knows its location and identity. In that event, the user terminal can provide service (make telephone calls) immediately, because the fixed site is already aware of the existence of the mobile user terminal within its broadcast cell. On the other hand, if the user has moved the mobile terminal to a new location, as by driving along a highway, the LAI codes at initial turn-on won't match, because some aspect of the currently received LAI will differ from the stored value. In other words, at least the LAC, which identifies the fixed site, will have changed, the provider may have changed, and if a national boundary has been crossed, the MCC may also have changed. In the event that the mobile user terminal finds that the stored and newly received LAI do not match, it begins an update procedure, by requesting access with the current serving cell. Once access is granted by assigning a carrier frequency and a time multiplexing pattern on that frequency, the user terminal automatically sends a location update request message. The network then examines the location update request message, to identify the user terminal's home network, and to verify the legitimacy of the user's service. This examination involves, inter alia, accessing a database at the user terminal's home network, and advising the home network of the new location of the user terminal. Once service or roaming is authorized, the network sends a Location Updating Accept signal to the user terminal, which signal contains the new LAI of the fixed site. The user terminal then updates its stored LAI value for future use, and is ready to provide service by way of the particular fixed site with which it is communicating.
In the GSM system, if a phone call is made to the user terminal's home network, requesting access to the particular mobile user terminal, the home network's database knows the current cell location of the particular user, and can direct the call to the appropriate fixed site, without polling all possible fixed sites to try to locate the user terminal.
It has been discovered that the above-described cellular communication system cannot be used for a spacecraft cellular communication system. More particularly, in a spacecraft-based cellular communications system, there are plural mobile stations which may move throughout the spacecraft coverage area, and there are also fixed ground-based stations or gateways, which allow the spacecraft to couple the signals from the mobile user terminals to stations on the terrestrial land-line system. In general, there are many such gateways, with plural gateways in at least some countries. For example, Indonesia is a nation which consists of an elongated chain of islands, and each island which requires service must have a ground-based gateway. In a spacecraft cellular communication system, each cell is associated with a particular antenna beam, all of which originate with the spacecraft. Consequently, when a mobile user terminal is initially energized, and scans frequency to determine what carrier signals it can receive, it finds that it can receive one or more control signal carriers, just as in the case of the GSM. The mobile user terminal then selects the best of the signals, and examines the LAI. Upon comparison of the LAI with the previous LAI, it may be found to be the same, or different. If the control signal carried or transmitted over one spot beam relates to a single one of the gateways, then a mobile user who is within a particular spot beam can communicate only with one gateway, namely that gateway which generates the control signals for that particular spot beam. This is very disadvantageous, because a mobile user who happens to be within a particular spot beam, and who wishes to communicate with a particular country, may have to communicate by way of a third country within which the gateway associated with "his" spot beam lies. Similarly, a land-line user in a particular country, wishing to communicate with a particular mobile user terminal, would have to route his telephone call to that one of the gateways, wherever located, which was associated with the spot beam in which the user terminal was located. The difficulty of routing calls in this fashion is apparent, especially considering that the terrestrial caller might not know exactly where the mobile user terminal was located. In the case of a terrestrial caller located on an island with a single gateway, communication could not be routed by land lines to another gateway because there are no such land lines, and so communication could not be established with a mobile user terminal which was not in the spot beam associated with his island's gateway.
Improved spacecraft cellular communications systems are desired.